The present invention relates to a magnetic resonance imaging apparatus (MRI apparatus) such as a diagnostic magnetic resonance imaging apparatus which utilizes a magnetic resonance effect for externally determining the density distribution, relaxation time distribution, and chemical shift of specific atomic nuclei (normally hydrogen nuclei) in various tissues of a living body, as a subject, in a noncontacting manner, thereby providing image information for medical diagnosis, and more specifically to a magnetic resonance imaging apparatus in which electromagnetic interference between inclined magnetic field coils and transmission and reception probe heads is reduced.
Conventionally, diagnostic magnetic resonance imaging apparatuses comprise a magnetostatic field coil, a probe head for transmission, a probe head for reception, and inclined magnetic field coils for X-, Y- and Z-directions. The magnetostatic field coil serves to generate a magnetostatic field in a predetermined direction (Z-direction). The transmission probe head applies a high-frequency magnetic field to a subject, while the reception probe head receives magnetic resonance signals from the subject. The inclined field coils generate pulsative inclined magnetic fields in three rectangular directions (X-, Y- and Z-directions) an applies them to the subject in order to discriminate the positions of signal generation and obtain a cross-sectional image of the subject.
In apparatuses of a type in which the direction (Z-direction) of the magnetostatic field (produced by the magnetostatic field coil) is in line with the body axis of the subject, the reception probe head and the inclined field coils for X- and Y-directions are each generally formed in an arcuate, saddle-shape. Since the respective arcuate portions of the saddle-shaped coils have their centers of curvature on the same axis, it is inevitable that the reception probe head and the X- and Y-direction inclined field coils are coupled electromagnetically. Therefore, the quality factor of the reception probe head is reduced, resulting in increased noise of the reception circuit. Thus, the apparatus cannot provide good quality cross-sectional image information, and fails to efficiently detect weak magnetic resonance signals from the subject.
The following two measures have conventionally been taken to meet this situation.
In a first system, four inclined magnetic field coils are arranged partially parallel to one another. As an example of this arrangement, FIG. 1 shows an apparatus stated in "NMR Imaging Biomedicine," Academic Press (1982), pp. 276, compiled by P. Mansfield and P. G. Morris.
In FIG. 1, transmission probe head 4, reception probe head 6, inclined magnetic field coil means 8 and 10 for the X- and Y-directions, respectively, and inclined magnetic field coil means (not shown) for the Y-direction are arranged inside magnetostatic field coil assembly 2.
Transmission probe head 4 is formed of a pair of saddle-shaped coils 4a, and reception probe head 6 also of a pair of saddle-shaped coils 6a. X-direction inclined field coil means 8 is formed of four rectangular coils 8a, which are arranged so that their respective inside portions are parallel to one another.
According to this configuration, electromagnetic coupling between coil means 8 and probe heads 4 and 6 can be avoided. However, the efficiency of generation of inclined magnetic fields is lower than that obtained with use of saddle-shaped coils. In apparatuses using superconductive coils, moreover, only a narrow space can be provided for the parallel location of the inside portions of coils 8a, thereby complicating coil arrangement.
In a second system, as shown in FIG. 2, electromagnetic coupling prevention member 22, i.e., a cylindrical high-conductivity member made of copper foil, is interposed between probe heads 12 and 14 (for transmission and reception, respectively), and inclined magnetic field coil means 16, 18 and 20 for X-, Y-, and Z-directions. According to this system, electromagnetic coupling between probe heads 12 and 14 and coil means 16, 18 and 20 can be reduced. However, the inclined magnetic fields, which vary pulsatively, produce eddy currents (indicated by arrow a) inside the high-conductivity member, as shown in FIG. 3, thus preventing quick rising and falling of pulses of the inclined fields.